Where to put freon in a 1999 Ford Mustang?

You should not attempt to add freon to your 1999 Ford Mustang's air conditioning system yourself. Adding refrigerant incorrectly can damage the system and potentially be hazardous. Freon (now mostly replaced by refrigerants like R-134a) is under high pressure and requires specialized equipment to handle safely and accurately.

Instead of trying to add freon yourself, take your Mustang to a qualified mechanic or an auto air conditioning specialist. They have the proper tools, knowledge, and safety equipment to diagnose the problem and recharge the system correctly. They'll be able to identify if it's a simple refrigerant leak or a more complex issue requiring repair.